1. Field of the Invention
The invention in general relates to steam turbine bypass systems, and more particularly to a control arrangement for preventing excessive temperatures in the high pressure turbine when the bypass system is in operation.
2. Description of the Prior Art
In a typical steam turbine power plant, a steam generator such as a boiler produces steam which is provided to a high pressure turbine to a plurality of steam admission valves. Steam exiting the high pressure turbine is reheated in a conventional reheater prior to being supplied to a lower pressure turbine, the exhaust from which is conducted into a condenser where the exhaust steam is converted to water and supplied to the boiler to complete the cycle.
With steam turbines equipped with a bypass system, the steam admission valves to the turbine may be closed, or partially closed, while still allowing steam to be produced by the boiler at a load level independent of steam turbine load. The bypass system is advantageously used for hot restarts or to keep the boiler on-line during plant or system transients that would normally require a trip (shutdown). Accordingly, bypass systems are provided in order to enhance on-line availability, obtain quick restarts, and minimize turbine thermal cycle expenditures.
In the operation of the power plant, a situation may arise wherein the electrical tie of the power line with a grid load network is interrupted. In such situations, it is still desirable to operate the steam turbine system at a house load level so as to supply the electrical needs of auxiliary equipment such as pumps, pulverizers, fans, etc. Under such conditions, the turbine will continue running at synchronous speed although with a greatly reduced steam flow and with the remainder of the boiler-produced steam being provided to the bypass system.
Ordinarily, sufficient steam flow must be passed through the turbine in order to keep the turbine elements cool. With the reduced flow rate conditions, however, a windage effect takes place whereby instead of extracting work from the steam, the turbine blades are actually doing work on the steam which is being churned up, resulting in a temperature increase which, in turn, causes the turbine parts to heat up. A danger therefore exists under such conditions, of turbine overheating past its design rating, thereby resulting in reduced life and possible premature failure.
The present invention provides for a significant improvement under such conditions whereby the turbine temperature is maintained within design limits.